Shaped articles based on pyrogenically produced titanium dioxide

ABSTRACT

Pressed parts based on pyrogenically produced titanium dioxide with the following physico-chemical characteristics: -Outer diameter 0.8-15 mm -BET surface according &lt;1-20 m2/g -to DIN 66 131 -Pore volume 0.01-0.29 cm3/g -Pore distribution no pores &lt;10 nm - at least 90% of the pores - in a range of 10-80 nm -Breaking strength 30-500 N/pressed part -TiO2 phase rutile 100% -Composition &gt;99.5% TiO2 -  They are produced by mixing pyrogenically produced titanium dioxide with urea, graphite and water, compacting the mixture, optionally drying the resulting mass at 80 DEG  C. to 120 DEG  C. comminuting, subsequently extruding or tabletting to produce pressed parts and tempering the pressed parts at a temperature of 710 DEG  C. to 1000 DEG  C. for a period of 0.5 to 6 hours. The pressed parts can be used as catalyst or catalytic carrier.

The present invention relates to shaped articles based on pyrogenically produced titanium dioxide, a method for producing them from pyrogenically produced titanium dioxide and their use as catalytic carriers or catalysts.

BACKGROUND OF THE INVENTION

Pyrogenically produced titanium dioxide is produced by means of high-temperature hydrolysis of a volatile titanium compound, usually titanium tetrachloride, in an oxyhydrogen flame. This method is described in German Patent DE-PS 870,242 (1953). The resulting products primarily exhibit an anatase structure. They are hydrophilic, very pure and extremely fine. The primary particles (DIN 53 206) exhibit a spherical form and diameters of 10-100 nm in electron-microscopical micrographs. A commercially available product exhibits an average primary particle size of 30 nm. Defined agglomerates (according to DIN 53 206) do not exist. The surface of the particles is smooth and free of pores. There is only an outer, readily accessible surface. The specific surface according to BET can be between 20 and 100 m² /g, depending on production conditions. The afore-mentioned commercial product exhibits a specific surface of 50±15 m² /g.

Because of its high purity, high specific surface and the lack of pores, pyrogenically produced titanium dioxide is used as a titanium-dioxide component or as carrier material in catalytic systems (V. Rives-Arnau, G. Munuera, Appl. Surface Sci. 6 (1980) p. 122; N. K. Nag, T. Fransen, P. Mars, J. Cat. 68, p. 77 (1981); F. Solymosi, A. Erdohelyi, M. Kocsis, J. Chem. Soc. Faraday Trans pp. 1, 77, 1003 (1981); D. G. Mustard, C. H. Bartholomew, J. Cat. 67, p. 186 (1981); M. A. Vannice, R. L. Garten, J. Cat. 63, p. 255 (1980), M. A. Vannice, R. L. Garten, J. Cat. 66, p 242 (1980) .

However, only powdery catalytic systems are discussed in the literature cited above- If pyrogenically produced titanium dioxide is to be used on an industrial scale in catalytic systems, it would be advantageous to convert the powdery product into shaped bodies.

Since pyrogenically produced titanium dioxide has an especially fine particle size, its formation into shaped catalytic carriers presents problems.

Published German Patent Application DE-OS 31 32 674 discloses a method for the production of pressed parts of pyrogenically produced titanium dioxide in which silica sol is used as binder. Ethylene glycol, glycerol, erytherites, pentitols or hexitols are used as auxiliary pressing agents. This method has the disadvantage that an undesired "covering" occurs in the pressed parts during large-scale production, that is, an outer layer separates from the pressed parts.

Moreover, when this method is used, the shaped bodies contain considerable amounts of SiO₂ in addition to TiO₂, which arises from the binding agent. This influences the effectiveness of the catalyst.

Published German Patent Application DE-OS 32 17 751 discloses the pressing of pyrogenically produced titanium dioxide in which organic acids or acidic-reacting salts are used as intermediate binding agents.

This method has the disadvantage that the nature of the surface of the pyrogenically produced titanium dioxide is changed by the treatment with organic acids or acidic-reacting salts.

Published German Patent Application P 38 03 894 also teaches titanium-dioxide pressed parts.

The known methods are disadvantageous because a binding agent must be used which remains in the pressed part. This leads to a change of the catalytic action of the very pure, pyrogenically produced titanium dioxide.

Methods using precipitated titanium dioxide have the disadvantage that impurities are introduced by the precipitation process which influence the catalytic action.

SUMMARY OF THE INVENTION

The object of the present invention is to provide pressed parts of pyrogenically produced titanium dioxide which retain the high purity of the starting material, that is, they do not contain residues of binding agents. Furthermore, the TiO₂ phase preferably should be in the rutile form.

The pressed parts based on pyrogenically produced titanium dioxide, according to the invention, have the following physico-chemical characteristics:

    ______________________________________                                         Outer diameter     0.8-15 mm                                                   BET surface according                                                                             <1-20 m.sup.2 /g                                            to DIN 66 131                                                                  Pore volume        0.01-0.29 cm.sup.3 /g                                       Pore distribution  no pores <10 nm                                                                at least 90% of the pores                                                      in a range of 10-80 nm                                      Breaking strength  30-500 N/pressed part                                       TiO.sub.2  phase   rutile 100%                                                 Composition        >99.5% TiO.sub.2                                            ______________________________________                                    

The outer diameter can preferably be 0.8 to 1.8 mm.

The length of the pressed parts can preferably be 1 to 15 mm.

The invention also provides a method for producing the pressed parts based on pyrogenically produced titanium dioxide with the following physico-chemical characteristics:

    ______________________________________                                         Outer diameter     0.8-15 mm                                                   BET surface according                                                                             <1-20 m.sup.2 /g                                            to DIN 66 131                                                                  Pore volume        0.01-0.29 cm.sup.3 /g                                       Pore distribution  no pores <10 nm                                                                at least 90% of the pores                                                      in a range of 10-80 nm                                      Breaking strength  30-500 N/pressed part                                       TiO.sub.2  phase   rutile 100%                                                 Composition        >99.5% TiO.sub.2                                            ______________________________________                                    

According to the method of the invention, pyrogenically produced titanium dioxide is mixed with urea, graphite and water, compacted, the mass obtained is optionally dried at 80° C. to 120° C. and comminuted, subsequently extruded or tabletted and the pressed parts obtained are tempered at a temperature of 710° C. to 1000° C. for a period of 0.5 to 6 hours.

In a preferred embodiment of the invention, the mixture can have the following composition before the pressing:

50-90% by weight titanium dioxide, preferably 65-85% by weight

5-50% by weight urea, preferably 15-30% by weight

0.1-8% by weight graphite, preferably 1-5% by weight.

In principle, all mixers or mills are suitable for carrying out the method of the invention, if they make it possible to achieve good homogenization, such as e.g. blade, fluid-bed, gyratory or air-swept mixers. Mixers with which an additional compression of the mixing material is possible are especially suitable, e.g. plowshare mixers, mulling machines, ball mills and extruders. After the homogenization, the mass can be directly extruded or it can be dried first, at 80°-120° C. The dry mass can be ground to a flowable powder. The drying and grinding of the voluminous and fine, pyrogenically produced titanium-dioxide powder, which are preferred measures, together with the auxiliary tabletting means and pore formers, result in a homogeneous, flowable powder which leads to shaped bodies with improved physical properties in terms of breaking strength and abrasion. The pressed parts can be produced in extruders, hand presses, eccentric presses or concentric-running presses as well as on compactors.

The breaking strength, specific total surface and the pore volume can be varied to some extent by varying the amounts of additives, the compression force and the tempering conditions.

The pressed parts of the invention can be produced in a variety of shapes such as, e.g., cylindrical, spherical, annular with an outer diameter of 0.8-15 mm.

The pressed parts of the invention can be used either directly as catalysts or as catalytic carriers after the shaped bodies have been impregnated, during or after their formation, with a solution of a catalytically active substance and optionally activated by means of a suitable post-treatment.

In particular, pressed parts of pyrogenically produced titanium dioxide can be used with especially good advantage as carriers or as catalysts in the Fischer-Tropsch synthesis and in the preparation of formaldehyde (dehydrogenation of methanol).

The pressed parts of the invention have the following advantages:

They have the same chemical composition as the starting material (pyrogenically produced titanium dioxide).

They do not contain any additives, such as binding agents, urea or graphite.

High Breaking Strength.

The titanium dioxide is present in the rutile form.

The predominant number of pores is in the mesopore range.

No pores>10 nm.

The pressed parts of the invention contain no additional substances because only urea and graphite are used during the production method as pore former and lubricant. However, both substances are removed after the shaping process during the tempering.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In the following examples:

The BET surface is determined according to DIN 66 131 with nitrogen.

The pore volume is calculated from the sum of the micro, meso and macropores.

The breaking strength is determined by means of the breaking-strength tester of the firm Erweka, type TBH 28. The pressing power is adjusted upward in a continuous manner via the tension of the motor until the shaped body (tablet, extrudate, sphere, etc.) breaks. The exerted force is indicated digitally at the moment of the break.

The determination of the micro and mesopores is made by recording an N₂ isotherm and evaluating it according to BET, de Boer and Barret, Joyner, Halenda. The determination of the macropores is made by the Hg injection method.

The titanium dioxide phase determination takes place by means of radiographs in a diffractometer.

Pyrogenically produced titanium dioxide P25 is used as titanium dioxide in the examples, which is characterized by the following physico-chemical characteristic data:

    ______________________________________                                                                TiO.sub.2                                                                      P 25                                                    Surface according to BET                                                                              m.sup.2 /g 50 ± 15                                   Average size of the primary                                                                           nm 30                                                   particles                                                                      Stamping density.sup.1) normal item                                                                   g/l approx. 150                                         Drying loss.sup.2) (2 hrs. at 105° C.)                                                         %<1.5                                                   Annealing loss.sup.2)5) (2 hrs. at                                                                    %<2                                                     1000° C.)                                                               pH.sup.3) (in 4% aqueous dispersion)                                                                  3-4                                                     SiO.sub.2.sup.6)       %<0.2                                                   Al.sub.2 O.sub.3.sup.6)                                                                               %<0.3                                                   Fe.sub.2 O.sub.3.sup.6)                                                                               %<0.01                                                  TiO.sub.2.sup.6)       %>99.5                                                  HCl.sup.6)8)           %<0.3                                                   ______________________________________                                          .sup.1) according to DIN 53 194                                                .sup.2) according to DIN 55 921                                                .sup.3) according to DIN 53 200                                                .sup.5) relative to the substance dried 2 hours at 105° C.              .sup.6) relative to the substance annealed 2 hours at 100° C.           .sup.8) HCl content is a component of the annealing loss                 

EXAMPLE 1

70% titanium dioxide P 25

25% urea

5% graphite

are compacted with the addition of water, dried at 100° C. for 24 hours and comminuted to a flowable powder.

The pressed parts are formed in an extruder.

The raw, extruded pressed parts are tempered at 720° C. for 6 hrs. The pressed parts obtained exhibit the following physico-chemical characteristics:

    ______________________________________                                         Outer diameter   1.2 mm                                                        BET surface according                                                                           12 m.sup.2 /g                                                 to DIN 66 131                                                                  Pore volume      0.23 ml/g                                                     Pore distribution                                                                               no pores <10 nm φ                                                          95% of the pores in a                                                          range of 10-80 nm φ                                       Breaking         32 N/extruded pressed part                                    strength                                                                       Bulk weight      880 g/l                                                       Composition      >99.5% TiO.sub.2                                              TiO.sub.2 phase  rutile 100%                                                   ______________________________________                                    

EXAMPLE 2

Mixing and production of the raw, extruded pressed parts according to Example 1.

The raw, extruded pressed parts produced in Example 1 are tempered at 800° C. for 4 hrs. The pressed parts obtained exhibit the following physico-chemical characteristics:

    ______________________________________                                         Outer diameter   1 mm                                                          BET surface according                                                                           5 m.sup.2 /g                                                  to DIN 66 131                                                                  Pore volume      0.09 ml/g                                                     Pore distribution                                                                               no pores <10 nm φ                                                          96% of the pores in a                                                          range of 10-80 nm φ                                       Breaking         46 N/pressed part                                             strength                                                                       Bulk weight      1,250 g/l                                                     Composition      >99.5% TiO.sub.2                                              TiO.sub.2 phase  rutile 100%                                                   ______________________________________                                    

EXAMPLE 3

Mixing and production of the raw, extruded pressed parts are according to Example 1.

The raw, extruded pressed parts of Example 1 are tempered at 900° C. for 2 hrs. The pressed parts obtained have the following physico-chemical characteristics:

    ______________________________________                                         Outer diameter   1 mm                                                          BET surface according                                                                           1 m.sup.2 /g                                                  to DIN 66 131                                                                  Pore volume      0.04 ml/g                                                     Pore distribution                                                                               no pores <10 nm φ                                                          97% of the pores in a range                                                    of 10-80 nm φ                                             Breaking         65 N/pressed part                                             strength                                                                       Bulk weight      1,450 g/l                                                     Composition      >99.5% TiO.sub.2                                              TiO.sub.2 phase  rutile 100%                                                   ______________________________________                                    

EXAMPLE 4

70% titanium dioxide P 25

25% urea

5% graphite

are homogenized in an asymmetric moving mixer.

The extruded pressed parts were produced on an extruder.

The raw, extruded pressed parts are tempered at 720° C. for four hours. The pressed parts obtained h ave the following physico-chemical characteristics:

    ______________________________________                                         Outer diameter   1.3 mm                                                        BET surface according                                                                           13 m.sup.2 /g                                                 to DIN 66 131                                                                  Pore volume      0.27 ml/g                                                     Pore distribution                                                                               no pores <10 nm φ                                                          94% of the pores in a range                                                    of 10-80 nm φ                                             Breaking         31 N/pressed part                                             strength                                                                       Bulk weight      780 g/l                                                       Composition      >99.5% TiO.sub.2                                              TiO.sub.2 phase  rutile 100%                                                   ______________________________________                                    

EXAMPLE 5

72% titanium dioxide

25% urea

3% graphite

are compacted with the addition of water, dried at 100° C. for 24 hrs. and comminuted to a flowable powder. The tabletting takes place by means of an eccentric press.

The raw tablets are tempered at 900° C. for 4 hours. The pressed parts obtained have the following physico-chemical characteristics:

    ______________________________________                                         Outer diameter   5 mm                                                          BET surface according                                                                           6 m.sup.2 /g                                                  to DIN 66 131                                                                  Pore volume      0.12 ml/g                                                     Pore distribution                                                                               no pores <10 nm φ                                                          95% of the pores in a range of                                                 10-80 nm φ                                                Breaking         470 N/pressed part                                            strength                                                                       Bulk weight      1,700 g/l                                                     Composition      >99.5% TiO.sub.2                                              TiO.sub.2 phase  rutile 100%                                                   ______________________________________                                     

What is claimed is:
 1. Shaped articles comprising pyrogenically produced titanium dioxide which have the following physico-chemical characteristics:

    ______________________________________                                         Outer diameter  0.8-15 mm                                                      Length          1-15 mm                                                        BET surface according                                                                          <20 m.sup.2 /g                                                 to DIN 66 131                                                                  Pore volume     0.01-0.29 cm.sup.3 /g                                          Pore distribution                                                                              no pores <10 nm,                                                               at least 90% of the pores                                                      in a range of 10-80 nm                                         Breaking Strength                                                                              30-500 N/pressed part                                          TiO.sub.2  phase                                                                               rutile 100%                                                    Composition     >99.5% TiO.sub.2                                               Form            cylindrical, spherical or annular.                             ______________________________________                                    


2. The shaped articles according to claim 1 wherein the BET surface according to DIN 66 131 is<1 m² /g.
 3. The shaped articles according to claim 1 wherein the outer diameter is 0.8-1.8 mm. 